This invention pertains to protective surface coatings commonly known as paint coatings. Protective surface coatings are organic compositions applied to substrates to form continuous films which are cured or otherwise hardened to provide protection as well as a decorative appearance to the substrate. Protective surface coatings ordinarily comprise an organic polymeric binder, pigments, inert fillers and other paint coating additives. The polymeric binder functions as an organic vehicle for the pigments, inerts, and other additives and further functions as a binder for the pigments and inert fillers in the cured or hardened paint film. Polymeric binders can be thermosetting binders based on coreactive binder components. Conventional thermosetting polymers often require high temperatures as well as external crosslinkers. Some crosslinkers, such as melamines in conventional industrial coatings or triglycidyl isocyanurate for powder coatings, can cause toxicity problems. The release of volatile by-products, such as caprolactam, from some of these materials can cause film defects, such as cratering and bubbling.
Powder coatings are organic powder coating which are dry, and essentially 100% non-volatile solids based on a polymeric film forming binder. Powder coatings can be clear coatings without opacifying pigments or pigmented coatings containing opacifying pigments. Protective surface coatings based on powder paints are particularly useful in view of environmental compliance in that powder paints do not contain organic solvents or emit organic compounds upon baking or heat curing. Powder paints are finely pulverized polymeric compositions in the form of dry free-flowing fine powder which melts and flows at elevated temperatures to produce a smooth coated surface when applied to a substrate.
Thermosetting acrylic polymer based powder coatings exhibit good exterior durability as well as detergent and alkali resistance. Acrylic powders exhibit good surface hardness and scratch resistance but generally are less flexible than polyester powder coatings. Functionally reactive acrylic copolymers can be cured or cross-linked with crosslinkers such as aliphatic diacids to obtain good film appearance. When a diacid crosslinker is used, the diacid has a melting point below the curing temperature required for the coating. However, the chain length of the diacid has been found to have a profound effect on coating appearance. If the chain length is too short, a low level of plasticizing crosslinker is required, resulting in poor film appearance. If the chain length is too long, compatibility with the polar acrylic resin is poor and problems such as hazing in the film can occur. The use of glycidyl functional acrylic copolymers cured with diacid crosslinkers in a liquid paint system is described in U.S. Pat. No. 2,857,354. In U.S. Pat. No. 3,781,380, use of these materials is described for powder coatings although the crosslinkers are aliphatic diacid compounds. U.S. Pat. No. 4,937,288 describes the use of acid functional polyesters in conjunction with glycidyl acrylic polymers, where the polyesters described are amorphous.
It now has been found that coating compositions containing a new type of crystalline polyester crosslinker comprising the reaction product of an aliphatic dicarboxylic acid and preferred aliphatic diols results in good cured film appearance and good physical properties. Useful crystalline polyester crosslinkers can include a dicarboxylic acid polyester-urethane crosslinker. It has been found that glycidyl functional acrylic powder coating formulations can be crosslinked with the crystalline polyester crosslinker to give films that possess good appearance and mechanical properties. These powder coatings exhibit good physical and chemical stability, and provide cured films with good appearance exhibiting improved physical properties and exterior durability. These and other advantages of the invention will become more apparent by referring to the detailed description of the invention along with the illustrated examples.